1. Field of the Invention
The present invention relates to an analog transmitter, and more particularly, to an amplitude shift keying (ASK) transmitter.
2. Description of the Related Art
Amplitude shift keying (ASK) is a modulation scheme in which an amplitude of a carrier wave varies according to digital signal information. For example, in the modulation scheme, in order to modulate a binary digital signal, a carrier wave having a predetermined level of the amplitude is transmitted when the digital signal has a value of 1, and transmission of the carrier wave is suspended when the digital signal has a value of 0. The ASK modulation scheme in which the carrier wave is present (ON) or absent (OFF) is called an on-off keying (OOK) modulation scheme.
A modulation index in a frequency modulation scheme is one of the most important electric characteristics to determine performances of a transmitter and a receiver along with a modulation factor in an amplitude modulation scheme. The modulation factor is defined as a ratio m/M of an amplitude m of a signal wave to be transmitted to an amplitude M of a carrier wave in amplitude modulation. Here, the amplitude M of the carrier wave means an amplitude of the carrier wave that is not modulated yet. The ratio m/M expressed as a percentage refers to a modulation rate. The modulation index in the frequency modulation scheme corresponds to the modulation factor in the amplitude modulation scheme and represents a degree or efficiency of modulation. As the modulation index decreases, carrier wave power (energy) increases, and modulated wave power (energy) decreases. Here, the modulation index and the modulation factor are used as the same.
FIG. 1 illustrates an amplitude modulation (AM) waveform when the modulation index m is 0.5 (referred to as 50%).
FIG. 2 illustrates an AM waveform when the modulation index m is 1 (referred to as 100%).
FIG. 3 illustrates an AM waveform when the modulation index m is greater than 1.
Referring to FIGS. 1 to 3, it can be seen that the modulation index m having a value of slightly smaller than 1 (referred to as 100%) is relatively effective.
However, the value of the modulation index is determined differently according to a communication system using the transmitter and the receiver, so that an optimal modulation index cannot be fixed. The value of the modulation index is determined when the transmitter and the receiver are manufactured and cannot be arbitrarily changed by a user. Therefore, there is a problem in that the transmitter and the receiver that are manufactured to be used for a particular communication system cannot be used for another communication system having different electric characteristics.